Design work on this new generation sidearm began in 1994, and the handgun was presented in 1997 with series production commencing that same year. The weapon was first introduced chambered in 9×19mm Parabellum.

The Walther P99 was modified several times throughout its production history, as were the magazines. When the P99 was introduced the magazine capacities were 16 rounds for 9×19mm Parabellum and 12 rounds for .40 S&W and the magazines had witness holes on both sides of the magazines. Later the magazine capacities were reduced to 15 rounds for 9×19mm Parabellum and 11 rounds for .40 S&W and witness holes at the rear of the magazine to view the loading condition were introduced.[5]

A redesigned second generation P99 was presented in 2004, incorporating a modified trigger guard that eliminated the "ski hump", which is clearly visible in the accompanying first generation images. This was done to address some users' comfort concerns regarding the previous style.[citation needed] The first generation P99 pistols had a slide release at the left side of the frame, second generation guns may have optional ambidextrous slide stop release levers. Walther also took the opportunity to redesign the slide so the user could grip it more easily, and notably, change the proprietary accessory rail to a Weaver type. Some models built in 2005 and all later models received one more design change, an elongated magazine release.[6]

Walther P99 with the slide locked back displaying its vertical barrel tilt.

The Walther P99 is a short recoil-operated locked breech semi-automatic pistol that uses a modified Browning cam-lock system adapted from the Hi-Power pistol. The P99 has a glassfiber-reinforced polymer frame and steel slide assembly. It can be broken down into its main parts or field stripped with a take down catch without the help of tools.

It uses an internal striker as opposed to an external hammer, with a red-painted striker tip that protrudes from rear of the slide when the gun is cocked, as well as a loaded chamber indicator on the right side of the slide.

The original, first-generation P99 had a traditional Double-Action/Single-Action trigger with a decocker. When the slide is racked completely to the rear upon loading, the trigger remains in the forward position for the first shot and the Anti-Stress mode is activated, where the striker is cocked but the trigger has the length of pull of the double-action mode. The trigger travel is 14 mm (0.55 in) long in the Anti-Stress mode, but the most of the length is slack and is similar in feel to a two-stage rifle trigger. At the end of the anti-stress travel, the pressure noticeably increases to 4.5 pounds and the user experiences a crisp trigger break and an extremely short trigger reset of about .1 inches. All subsequent shots are fired in standard single action mode, where the slack on the trigger is much shorter, but the point of release feels the same. In order to deactivate Single-Action or Anti-Stress mode, the user can manipulate the decocker button on the top of the slide, just in front of the rear sights. When decocked from Single-Action mode, the trigger will reset to the full length of its pull. The pistol is now in Double-Action mode with a pull 7.9 pounds for the full length of the .55 inch pull. After firing the first shot in Double-Action, the pistol will be fired in Single-Action for all subsequent shots.

The striker protrudes visibly and palpably from the back of the slide when the firearm is in a constant partially cocked state. It should however be noted that the striker of the second generation P99QA variant does not protrude from the back of the slide, unless fired, as the firearm is in a constant partially cocked state.

Ergonomics were a key focus in the design of the firearm, and as a result, three interchangeable grip backstraps are included (small, medium and large) to accommodate various hand shapes and sizes; this feature permits most shooters a comfortable and efficient grip on the firearm. This adaptable grip was innovative at the time the P99 was introduced.

The slide and other metal parts of the pistol are Tenifer treated (a nitriding process also used on Glock pistols). The Tenifer finish is between 0.04 mm (0.002 in) and 0.05 mm (0.002 in) in thickness, and is characterized by extreme resistance to wear and corrosion; it penetrates the metal, and treated parts have similar properties even below the surface to a certain depth.[7] The Tenifer process produces a matte gray-colored, non-glare surface with a 64 Rockwell C hardness rating and a 99% resistance to salt water corrosion (which meets or exceeds stainless steel specifications),[8] making the P99 particularly suitable for individuals carrying the pistol concealed as the highly chloride-resistant finish allows the pistol to better endure the effects of perspiration.[7]

The P99 feeds from staggered-column or double-stack magazines of varying capacity. Walther also offers staggered-column magazines with a +2 baseplate that add 2 rounds to the magazine capacity at the expense of extending the grip and the pistol height by approximately 18 mm (0.7 in).

The .40 S&W caliber models incorporate a slightly larger slide in order to preserve the same recoil spring assembly used in its 9mm counterpart. The P99 is available in 4 colorations; a black frame with a black slide, a black frame with a titanium-coated slide, a military olive-drab frame with a black slide, and a desert tan frame with black slide. All three of these finishes incorporate the same black grip inserts that come with every P99. The compact versions of the P99 are only available in all black.[9]

This variant comes closest to a traditional Double-Action/Single-Action trigger with a decocker. In Double-Action mode the internal striker is at rest with no pre-load and the trigger travel is 14 mm (0.55 in) long at approximately 38 N (8.5 lbf) trigger pull. When the slide is racked completely to the rear upon (re)loading, the internal striker is manually pre-cocked to Single-Action mode and the trigger remains in the forward position for the first shot and the Anti Stress mode is activated. The trigger travel is 14 mm (0.55 in) long in the Anti Stress mode at approximately 20 N (4.5 lbf) trigger pull. On all subsequent shots, the P99AS is automatically pre-cocked and the trigger travel is reduced to 8 mm (0.3 in) at the same 20 N (4.5 lbf) trigger pull. For manually deactivating the Anti Stress mode and setting the pistol to Double-Action mode the P99AS features a decock key or knob on the slide. The Anti Stress mode can also be manually activated by moving the slide approximately 10 mm (0.4 in) back.[11]

The P99DAO is similar to the previously produced P990 Double Action Only variant.[12] The internal striker is at rest with no pre-load. The trigger travel is approximately 14 mm (0.55 in) with a trigger pull of approximately 38 N (8.5 lbf). The trigger pull is consistent in length and force from the first shot to the last and the striker returns to its de-cocked position after each shot.[11]

This variant has a Glock style trigger system with a pre-loaded internal striker. When the trigger is pulled, the striker is fully cocked and released, firing the pistol. The trigger travel is approximately 8 mm (0.3 in) with a trigger pull of approximately 38 N (8.5 lbf). The trigger pull is consistent in length and force from the first shot to the last.[11] The P99QA was announced in 2000 and discontinued in 2011.[citation needed]

The P99RAM is a CO2 training and simulation pistol, equipped with a CO2 blow-back system. It fires low-cost Paintball- and Rubberball ammunition in caliber .43 in (11 mm). In its design, controls and handling, the P99RAM is indistinguishable from the original P99 pistol. Lightweight protective clothing is needed to adequately protect the users for Paintball- and Rubberball simunition hits during training.

Year 2000 - a limited edition P99 fabricated to commemorate the new millennium. 2000 of these were made to reciprocate the year for which it was issued.

P99 TA - a model prepared for the police tryouts in Baden-Württemberg in 2002, featuring a single/double-action trigger with redesigned trigger guard, center location ambidextrous decocker and an early ambidextrous slide catch as well as the removal of the "captured" accessory rail in favor of a standard "open" rail system. Lastly, this model has new contours on the lower frame just ahead of the disassembly clasp, just before the accessory rails to facilitate holstering.[13] Approximately fifty (50) P99 TAs were produced with less than 25 sold to the public (10 imported to the U.S.).[citation needed]

The P99 RAD variant is made under license by Fabryka Broni Radom in Poland and marketed as a military sidearm proposition.[14]

The P99 RAD grip shape and texture has been altered when compared to the P99 variants and an extended ambidextrous slide release and extended ambidextrous magazine release levers are fitted as a standard feature. Besides that the slide shape, dimensions and serrations have been altered. The standard sights are high-contrast 3 dot sights with contrast enhancements that have been painted with afterglow paint that can aid target acquisition under unfavourable lighting conditions. As an option the P99 RAD can also be fitted with self-luminous tritium night sights. The P99 RAD pistol is available with the DAO trigger mechanism with a trigger pull of approximately 39 N (8.8 lbf) and the QA trigger mechanism with a trigger pull of approximately 32 N (7.2 lbf). The grip frame has an integrated mounting MIL-STD-1913 (Picatinny) rail for attaching tactical lights and laser sighting devices.

Many of the alterations found in the P99 RAD variant were applied in the Walther PPQ pistol introduced in 2011.

Many of the alterations found in the Fabryka Broni Radom P99 RAD variant were also applied in the Walther P99Q pistol. The P99Q is a designated police pistol, which has been certified according to the Technical Specifications (TR) of the German Police (Technische Richtlinie Pistolen im Kaliber 9mm x 19, Revision January 2008).[15] The P99Q is exclusively manufactured for German and foreign police departments and is not available on the civilian market.[16] The P99Q has a partially pre-loaded internal striker trigger mechanism that conforms to the Technical Specifications (TR) of the German Police. When the trigger is pulled, the striker is fully cocked and released, firing the pistol. Trigger travel is approximately 13 mm (0.5 in) with a trigger reset of 6 mm (0.2 in) and a trigger pull of approximately 32 N (7.2 lbf). The trigger pull is consistent in length and force from the first shot to the last. The Technical Specifications (TR 2008) of the German Police also demand an accuracy of fire for a 10-shot group with German Police certified 9×19mm Parabellum ammunition[17] at 25 m (27 yd) (R100)[18] of ≤ 16 cm (6.3 in). This equals an accuracy of fire of ≤ 22 minute of angle (MOA) or ≤ 6.4 mrad. The grip frame has an integrated mounting MIL-STD-1913 (Picatinny) rail for attaching tactical lights and laser sighting devices.

The Smith & Wesson SW99 is a joint venture between Walther, who produces a modified receiver in Germany, and Smith & Wesson, who fabricates the slides and barrels in the United States.[citation needed] Magazines can be interchanged between the two models, but the pistols are considered to be two separate types, and are easily distinguished from each other by the trigger guard, grip and slide design. The SW99 is also available in .45 ACP, while the P99 is not. The P99 was developed and introduced prior to the SW99. Walther introduced a color-coding system to denote the difference between early generation SW99 and P99 magazines, with the latest generation being bright blue.[citation needed]

All 9mm P99s have always been completely German-made. Some P99 .40 components are manufactured under license by S&W. The barrel, slide and frame of all 100% German-manufactured P99s bear an "Eagle over N" proof mark of the German Proof House at Ulm. The mark further indicates the pistol was test-fired with a Proof Load; the "N" stands for nitrocellulose.

The Magnum Research MR Eagle is another joint American/German venture, with Walther producing a black polymer frame with integral steel rails similar to the P99 series, only differing in grip texture. For its part, Magnum Research Inc. produces a 416 stainless steel slide and 4140 CrMo steel barrel in their Pillager, MN factory in the United States. The MR Eagle series pistols are available in 9×19mm Parabellum (MR9) and .40 S&W (MR40).[19][20]